The present invention relates to optical waveguides, and more particularly to means for minimizing the degeneration of signals transmitted to discrete modes along such waveguides.
For many years it has been recognized that light can be transmitted along strands of transparent material through the mechanism of internal reflection. This phenomenon is easily explained in a "step" waveguide, for example according to geometrical optics inasmuch as light rays reflect from the inner surface of the optical material, or more precisely from the interface of the core and an outer cladding having a smaller index of refraction. Also according to geometrical optics, if the light rays strike the outer surface of the transmitting member at some angle less than a predetermined maximum angle they will be continuously re-reflected along the waveguide with a minimal loss in energy. The principal loss of energy which is encountered by the light rays is simply attributable to the transmissivity of the medium.
More recently it has been discovered that elongate strands of light-transmissive material of extremely small cross-sectional size can be used to propagate light energy in various discrete modes, analogous to the transmission of microwaves. More particularly, when the radius of a glass waveguide is small compared to the wavelength of light being transmitted, for instance less than approximately ##EQU1## where .lambda. is the wavelength of the light, n is the refractive index of the core and .DELTA. is the fractional index difference between the core and the cladding, the light energy will be propagated in the structure as a single electromagnetic mode. More generally, for a given waveguide the light energy will no longer propagate in the waveguide core but will radiate from the waveguide and be lost. A detailed treatment of modal propagation may be found in "Fiber-Optics--Principles and Applications" by N. S. Kapany, published by the Academic Press of New York, N. Y.
Owing to the relatively small attenuation of the light energy in discrete modes, it is desirable to cause light energy to remain in these modes. Owing to the great number of available modes in practically-sized waveguides, when the light energy traversing the guide is modulated in order to transfer information through the guide the modulated information flows in at least several modes simultaneously.
Owing to the fact that the velocity of propagation of transmitted light is different for each mode, and that light energy introduced into the waveguide will be divided among the various modes, a light pulse entering the waveguide at one end will be seen to divide or "stretch" as it travels down the waveguide, exiting as a wider, or less well defined signal. This corresponds to a reduction in bandwidth of the signal transferring mechanism, and may be conceived of as resulting from the arrival of different elements of the transmitted pulse at different times.
In order to provide substantially equal transmission time for a signal propagated by different modes, it has been postulated that the signals in each mode should be periodically transferred or "coupled" to other modes.
A simplified illustration of this phenomenon would be as follows: consider several automobiles starting together, but in different lanes of a multi-lane highway. The speed allowed each vehicle is different for each lane of the highway, so that when the end of the highway is reached vehicles in the progressively lower speed lanes will arrive at progressively later times.
Now suppose that at relatively frequent intervals the vehicles are caused to switch lanes in substantially random fashion. At the end of a long highway each vehicle would have traversed each of the different speed lanes for a substantially equal amount of time, so that the average speed of each vehicle would be substantially the same. Accordingly, the vehicles would now reach the end of the highway at substantially the same time.
According to optical waveguide theory, the highway lanes may be analogized to light transmission modes. By causing light energy in each mode to repeatedly transfer to some other mode, the "mixing" of transmission modes will cause modulations of the light energy to propagate down the waveguide at a substantially uniform velocity. Hence, a modulation or signal traveling by several different modes will arrive at the distal end of a waveguide substantially simultaneously. This will substantially eliminate the stretching or degeneration of the modulation or, in other words, improve the signal bandwidth of the waveguide.
It is known that the intermixing, or coupling, of light energy between the modes along a length of waveguide will effect this result. It is also known that practically any sort of perturbation or variance of waveguide parameters will cause scattering or coupling of modes, whereby energy in a first mode will be converted to a second mode, and conversely. (See, e.g. "Theory of Dielectric Optical Waveguides" by D. Marcuse, published by the Academic Press.) The relatively uncontrolled nature of the coupling ordinarily provided, however, causes the energy in the desirable (bound or guided) modes to be converted or coupled into undesired (i.e. unbound or radiation) modes which effects a loss of light energy.
In an effort to minimize the coupling of light energy to the radiation modes it has been suggested that perturbations be formed in a waveguide by randomly-generated signals, or "noise." One example of an application of this theorem can be found in U.S. Pat. No. 3,687,514--Miller et al. The Miller et al patent suggests that the randomness of the scattering or coupling of the energy modes can be minimized by in effect filtering the noise, cutting off undesirably high frequency signals. Statistically, this is intended to effect coupling only among bound modes. However, the coupling is still not fully controlled and moreover a prescribed degree of coupling among only predetermined, bound modes is difficult to achieve.
In the above-noted Miller et al patent it is further disclosed that the periodic variation in waveguide parameters may consist of substantially the sum of all beat wavelengths taken in pairs, the variation to be repeated along the length of the waveguide. Since the number of frequencies required to couple all of the modes is very large, however, it would be extremely difficult to produce even one such periodic variation, much less an indefinitely long train of such perturbations repeated along the length of the waveguide.
Another example of an application of the general principle is found in U.S. Pat. No. 3,666,348--Marcatili. Marcatili as well as Miller et al suggest that sinusoidal perturbations may be formed along the axis of the waveguide, the frequency of the perturbations corresponding to the difference in propagation constants between the bound modes of the light propagating within the waveguide. It will be appreciated, however, that most waveguides are not identical and accordingly the necessary perturbation frequencies must be individually tailored for each individual waveguide.
For the foregoing reasons, it will now be appreciated that it would be desirable to provide a method for effecting a prescribed coupling of propagated light signals in waveguides, whose properties may vary from one to another, in order to achieve the desired coupling among certain bound transmission modes.
It is therefore an object of the present invention to provide a method for effecting the coupling of signals in an optical waveguide between only bound modes.
It is another object of the invention to provide a method of controlling coupling occurring between predetermined, bound light energy modes.
Yet another object is to achieve substantially uniform coupling among various, bound light energy modes.
Another object of the invention is to achieve a predetermined coupling of bound light energy modes in a manner independent of variations in modal propagation constants within and between different waveguides exhibiting substantially the same number of modes.
Still another object is to provide a predetermined coupling of bound light energy modes by forming identical perturbation functions at regular intervals along a waveguide.